Security researchers are seeing a resurgence of Gumblar, the name for a piece of malicious code that is spread by compromising legitimate but insecure Web sites.

In May, thousands of Web sites were found to have been hacked to serve up an iframe, which is a way to bring content from one Web site into another. The iframe led to the "gumblar.cn" domain. Gumblar would then try to exploit the user's PC via software vulnerabilities in Adobe Systems products such as Flash or Reader and then deliver malicious code.

Gumblar has also now changed its tactics. Rather than hosting the malicious payload on a remote server, the hackers are now putting that code on compromised Web sites, vendors IBM and ScanSafe say. It also appears Gumblar has been updated to use one of the more recent vulnerabilities in Adobe's Reader and Acrobat programs, according to IBM's Internet Security Systems Frequency X blog.

The hackers know that it's only a matter of time before a malicious domain is shut down by an ISP. The new tactic, however, "gives them a decentralized and redundant attack vector, spread across thousands of legitimate websites around the world," IBM said.

To help avoid detection, the bad code that is uploaded to the legitimate Web sites has been molded to match "existing file structures," IBM said. It also has been scrambled or obfuscated to try and avoid detection.

"Gumblar is a force to be reckoned with, and this latest push of theirs is a true testament to that fact," IBM said.

The hackers behind Gumblar have also taken to forcibly injecting a malicious iframe into forums, according to a blog post from ScanSafe. It means that people become victim to a so-called drive-by attack, where they are instantly exposed to malicious content from elsewhere when visiting a legitimate site.

Once a PC is infected, Gumblar also looks for other FTP credentials that it can use to compromise other Web sites. It also hijacks the Internet Explorer browser, replacing Google search results with those of other Web sites, which IBM thinks may be connected with "pay-per-click" fraud.